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Abstract:

Investigation of variations in the micro-texture and chemical composition of plagioclases (core to rim) allows the sequencing of the magma chamber processes and helps interprete and associate textures to specific processes. In this contribution, the micro-textures and chemical zoning of the plagioclases, significant recorder of magma chamber processes, from the west of Torbat-e-Heydariyeh andesitic rocks (WTHAR) are considered to decipher the physical and chemical parameters of magma evolution. The rocks are cropped out in the northern branch of Neotethyan magmatic belt and considered as the products of Eocene magmatic activities of the Sabzevar zone (N-NE Iran). The rocks show vitrorphyric and vitroglomeroporphyric textures with main phenocrysts, plagioclase (andesine, labradorite, and bytownite), clinopyroxene (augite), orthopyroxene and magnetite scattered in a glassy matrix. The recognized micro-textures of the WTHA plagioclases can be divided into two categories: (i) growth- related textures in the form of coarse-sieved (CS), fine-sieved (FS), core sieved and intact margin (CSIM), core intact and sieved margin (CISM) and entirely sieved (ES) morphologies, oscillatory zoning (OZ), rounded zone corner (RZC) and resorption surfaces (RS) formed due to the changes in temperature, melt H₂O content, pressure, composition of the melt, equilibrium at the crystal-melt interface and (ii) morphological textures such as glomerocrysts (GLO), synneusis (SY), swallow-tail (ST) crystals, broken crystals (BC), formed by the effect of dynamic behavior of the crystallizing magma (convection, degassing, etc.) and magmatic differentiation. Also, the occurrence of these changes can be related to the self-mixing process in the magma chamber. The self-mixing with recharge event can be the reason for the dynamic activities in the magma chamber.